1. Field of the Invention
The present invention is directed to a power amplifier of the type suitable for use in an NMR tomography apparatus, the power amplifier being of the type having a voltage source and a resonant assembly connected in parallel with the voltage source, the resonant assembly including a resonant capacitor connected in series with a control element.
2. Description of the Prior Art
An NMR tomography apparatus usually has three gradient coils in which precisely controlled periodic currents flow. The current behavior curve is dependent on the imaging method used. In a high-speed method, for example, as the EPI (echoplanar) method, it is particularly necessary to generate very steep and rapid consecutive leading and trailing edges of current. It thus can be necessary to build up and dismantle a current flow of 200 A in a gradient coil (with an inductivity of 1 mH, e.g.) within time intervals of 0.5 ms. The EPI method is described in detail in European Application 0 076 0054.
To satisfy these high demands, it is known to use a resonant circuit formed of the gradient coil and a capacitor. The gradient currents are sinusoidal at least in sections thereof.
European Application 0 227 411 discloses a gradient amplifier of the general type initially described. A gradient coil is arranged in a shunt arm of a circuit bridge. A resonant capacitor is connected in parallel with the circuit bridge via a switch element. A constant voltage is applied at the circuit bridge via a diode.
U.S. Pat. No. 4,628,264 discloses a gradient amplifier wherein a resonant capacitor and a gradient coil form a parallel resonant circuit. A switch element or a switch bridge is arranged in the resonant circuit in order to separate or change the polarity of the connection between the resonant capacitor and the gradient coil. Amplifiers which are permanently connected with the resonant capacitor are provided for compensating losses during the resonant oscillations.
German OS 41 27 529 discloses a gradient amplifier wherein a resonant capacitor is arranged in a shunt arm of a switch bridge. The switch bridge, a gradient coil and an amplifier assembly together form a serial resonant circuit. In the operation of the gradient amplifier, the amplifier assembly serves to generate individual sections of each gradient pulse in a non-resonant fashion.
The circuits according to European Application 0 227 411 and the U.S. Pat. No. 4,628,264 are able to generate only a limited number of different current profiles. Thus these circuits are not universally usable. Moreover, the embodiments with switch bridges result in a high component outlay. The circuit proposed in German OS 41 27 529 is likewise very expensive, because another switch bridge is required in addition to a complete amplifier assembly, typically constructed as a switched amplifier.